If $f(x) = \frac{1 + x}{1 - 3x}, f_1(x) = f(f(x)), f_2(x) = f(f_1(x)),$ and in general $f_n(x) = f(f_{n-1}(x)),$ then $f_{1993}(3)=$

Solution: $f(3) = \frac{1 + 3}{1 - 3\cdot 3} = -\frac{1}{2}$. Then $f_1(3) = f(-\frac12) = \frac{1 - \frac12}{1 + 3\cdot\frac12} = \frac15$, $\displaystyle f_2(3) = f(\frac15) = \frac{1 + \frac15}{1 - 3\cdot\frac15} = 3$ and $f_3(3) = f(3) = \frac{1 + 3}{1 - 3\cdot 3} = -\frac{1}{2}$.
It follows immediately that the function cycles and $f_n(3) = -\frac12$ if $n = 3k$, $f_n(3) = \frac15$ if $n = 3k + 1$ and $f_n(3) = 3$ if $n = 3k + 2$. Since $1993 = 3\cdot 664 + 1$, $f_{1993}(3) = \boxed{\frac{1}{5}}$.